Collaborative Research: Testing the fungal loop hypothesis for Carbon and Nitrogen cycling in dryland ecosystems

在森林和草地中，腐烂的植被在土壤表面积累，并被分解微生物群落消化。 分解的最终产物作为土壤中的养分，沿着水分，可以直接通过植物的根系被植物吸收。 相比之下，沙漠和其他干旱生态系统必须遵循一套不同的规则。 缺水意味着植物的分布更加零散，并且在罕见的降水事件之后短暂地生长。 在植物斑块之间，通常形成由表层细菌、真菌、地衣和苔藓组成的外壳。 土壤结皮真菌有称为菌丝的延伸，可以在结皮区域和植物之间建立联系。 该项目将研究“真菌环假说”的含义，该假说假定地下真菌菌丝在植物和土壤结皮之间提供了一个网络，可以保存和运输水分和养分到植物。 为了验证这一假设的各个方面，该项目的研究人员将在三个不同的地点进行实地研究：德克萨斯州埃尔帕索附近的奇瓦瓦沙漠，犹他州摩押附近的科罗拉多高原，以及新墨西哥州阿尔伯克基附近的一个地点。在这些地点，他们将研究水分和营养物质通过真菌菌丝的运动，并建立一个框架，以了解真菌循环在何时何地最重要。旱地占地球表面的40%左右，在地球对环境变化的总体反应中发挥着至关重要的作用。该项目的多地点、实地密集设计还将使本科生和研究生有机会在两个不同的机构进行研究和培训：得克萨斯大学埃尔帕索分校（UTEP）和新墨西哥州大学（UNM）。利用在三个地点进行的一系列田间试验，本项目将解决三个问题：（1）如何在旱地地点、植物物种和生物结壳类型之间改变转运率（即通过真菌菌丝在植物和生物结壳之间转移C和N）？(2)植物和生物结皮的碳、氮转运能促进生长、生产力和碳、氮的保持吗？(3)植物和生物结皮的化学计量需求决定了转运率吗？拟议的工作将产生一个预测框架，用于植物和生物结壳之间的C和N易位何时何地最大，方法是在每个地点使用各种植物和生物结壳官能团中的同位素示踪剂检查易位率（例如，C3与C4草），并结合季节变化，特别是对比春季和季风生长季节。这项工作还将通过实验切断菌丝连接和测量对植物和生物壳健康的影响以及生态系统中C和N的保留来研究易位的重要性。最后，为了解决易位的机制，研究人员将通过实验操作C和N梯度并观察对C和N水平运动的影响，以及使用同位素示踪剂来测试化学计量梯度驱动C和N运动通过真菌菌丝的假设。这种研究方法将允许在何种程度上真菌是旱地土壤中的C和N循环的关键调节因子的真菌循环假说所建议的一个前所未有的评估。

In forests and grasslands, decaying vegetation accumulates on the soil surface and is digested by communities of decomposer microorganisms. The end-products of decomposition serve as nutrients in the soil that, along with water, can be taken up by plants directly through their root systems. By contrast, deserts and other arid ecosystems have to play by a different set of rules. The lack of water means that plants are far more patchy in their distribution and grow in brief spurts following rare precipitation events. Between plant patches, a crust often forms consisting of surface-layer bacteria, fungi, lichens, and mosses. Soil crust fungi have extensions called hyphae that can make connections between crusted areas and plants. This project will examine implications of the "fungal loop hypothesis", which posits that subsurface fungal hyphae provide a network between plants and soil crusts that conserves and transports water and nutrients to plants. To test aspects of this hypothesis, researchers on this project will conduct field research at three different sites: the Chihuahuan Desert near El Paso, TX, the Colorado Plateau near Moab, UT, and a site between those, near Albuquerque, NM. At these sites, they will study the movement of water and nutrients through fungal hyphae and develop a framework for understanding when and where the fungal loop is most important. Drylands cover about 40% of Earth's surface and play essential roles in the planet's overall response to environmental change. The multi-site, field-intensive design of this project will also enable research and training opportunities for undergraduate and graduate students at two diverse institutions: the University of Texas at El Paso (UTEP) and the University of New Mexico (UNM).The overall objective of this study is to test the fungal loop hypothesis by studying C and N translocation and retention across representative dryland sites. Using a set of field experiments at three sites, this project will address three questions: (1) How do translocation rates (i.e. transfer of C and N between plants and biocrusts through fungal hyphae) vary among dryland sites, plant species, and biocrust types? (2) Does translocation improve growth, productivity and retention of C and N for plants and biocrusts? (3) Are translocation rates determined by the stoichiometric requirements of plants and biocrusts? The proposed work will generate a predictive framework for when and where translocation of C and N between plants and biocrusts is greatest by examining translocation rates using isotopic tracers in a variety of plant and biocrust functional groups at each site (e.g., C3 vs. C4 grasses) and incorporating seasonal variation, especially to contrast spring and monsoonal growing seasons. The work will also examine the importance of translocation by experimentally severing hyphal connections and measuring the effects on plant and biocrust health as well as retention of C and N in the ecosystem. Finally, to address the mechanism of translocation, the investigators will test the hypothesis that stoichiometric gradients drive C and N movement through fungal hyphae by experimentally manipulating C and N gradients and observing the effects on the horizontal movement of C and N, also with the use of isotopic tracers. This research approach will allow for an unprecedented evaluation of the extent to which fungi are the key regulators of C and N cycling in dryland soils as suggested by the fungal loop hypothesis.